. . . probably it may be taken generally that no spot of lesser diameter than 1′ of arc can be so seen. This measurement must be deemed to apply to that central portion of a normal spot already mentioned as being what is called the nucleus, because penumbræ may be more than 1′ in diameter without being visible to the naked eye, for the reason that their shading is so much less pronounced than the shading of umbræ. Very large and conspicuous spots are comparatively rare, though during the years 1893 and 1894 there were an unusual number of such spots. It often happens that a conspicuous group is the result of the merging or joining up of several smaller groups. In such cases a group may extend over an area on the Sun 3′ or 4′ of arc in length by 2′ or 3′ in breadth. The largest spot on record seems to have been one seen on September 30, 1858, the length of which in one direction amounted to more than 140,000 miles.

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